In the past, polyurethane compositions based on an isocyanate-terminated prepolymer comprising the reaction product of a polyol and a polyisocyanate cured with one or more polyfunctional crosslinking agents have been described in the art. Of particular concern herein are polyurethanes based on prepolymers comprising the reaction product of long chain fatty acid esters such as castor oil with organic polyisocyanates.
For example, in U.S. Pat. No. 3,362,921 to Ehrlich et al, curing agents for prepolymers based on the reaction product of active hydrogen-containing compounds such as castor oil, polyester amides and polyalkylene ether glycols with organic diisocyanates are described. These agents are esters of polyhydric alcohols containing at least four hydroxy groups and an aliphatic acid having at least 12 carbon atoms and one or more hydroxy and/or epoxy groups. The cured polyurethanes find use as flocking adhesives, paper coatings, potting compositions and encapsulating compounds for electronic parts.
U.S. Pat. No. 3,483,150 to Ehrlich et al. discloses prepolymer compositions which are the reaction product of at least one polyfunctional compound containing active hydrogens with an arylene diisocyanate and a low viscosity or solid polyfunctional isocyanate derived from the reaction of aniline and formaldehyde and having a functionality of 2 or greater, preferably between 2 and 3. The prepolymers are cured to elastomers by adding to the prepolymer at least one curing agent comprising a material containing two or more active hydrogen groups. Such curing agents include the curing agent of U.S. Pat. No. 3,362,921 and in addition, a glycol, glycerol, polyglycol, or polyalkylene glycol mono- or di-ester of a hydroxy carboxylic acid having at least 12 carbon atoms. Certain amines are useful in curing the prepolymers and include primary and secondary aliphatic, cyclic, aromatic, aralkyl and alkaryl diamines.
In U.S. Pat. No. 3,962,094 to Davis, a hollow fiber separatory device useful for dialysis, ultra-filtration, reverse osmosis, hemodialysis, etc., is provided. This device consists of a plurality of fine, hollow fibers whose end portions are potted in a tube sheet and whose open fiber ends terminate in a tube sheet face which provides liquid access to the interior of the fibers. The tube sheet comprises a cured polyurethane consisting essentially of a prepolymer based on the reaction product of castor oil with at least one mole per castor oil hydroxy group of an organic diisocyanate and crosslinked with either castor oil or an ester of a polyhydric alcohol having a hydroxyl functionality of 4 or more and an organic acid containing at least 12 carbon atoms and one or more hydroxy and/or epoxy groups per molecule, or mixtures of castor oil and the such esters.
Patents representative of the art of hollow fiber separatory devices include U.S. Pat. Nos. 2,972,349; 3,228,876; 3,228,877; 3,339,341; 3,442,088; 3,423,491; 3,503,515; and 3,551,331: the disclosures of which are incorporated herein by reference.
The sealing collar is typically derived from a resin which is capable of encapsulating the fibers to provide a seal which prevents the fluid inside the hollow fibers from mixing with the fluid outside the fibers.
A preferred class of resins useful for preparing the sealing collars are flexible polyurethane forming systems as illustrated by U.S. Pat. Nos. 3,362,921; 3,708,071; 3,722,695; 3,962,094; 4,031,012; 4,256,617; 4,284,506; 4,332,927 and Re. 31,389. Centrifugal casting, as illustrated by U.S. Pat. No. 3,492,698, is a representative method employed for preparing sealing collars. In accordance with such a technique, a holding device containing a bundle of fibers arranged in a parallel configuration is placed into a centrifugal-like device which incorporates a potting material reservoir with tubes connecting it to end-molds. An appropriate resin is placed into the potting reservoir and maintained at an appropriate temperature. The entire assembly is then rotated to force the resin down the connecting tubes by the centrifugal force. The resin thereby flows around and among the fibers in the end-molds. The rotation is continued until the resin gels. When polyurethanes are employed as the resin, typical residence time in the centrifuge can vary from about 1 to about 2 hours at room temperature. When rotation is completed the resin impregnated fiber bundle is removed and post-cured. The end molds are then removed and the fiber ends are opened by cutting through the resin collar perpendicular to the fiber bundle.
Other sealing collar forming techniques rely on the force of gravity to force the resin into a mold containing the ends of the hollow fibers. The resin is allowed to gel and then is post-cured.
Regardless of the particular method employed for preparing the sealing collar, the polyurethanes typically employed therein exhibit extended gel and demold times.
In addition to hollow fiber separatory devices, folded membrane separatory devices have also been used in chemical separations such as dialysis, osmotic processes and hemodialysis. In a folded membrane artificial kidney, for example, a membrane sheet is multiply-folded or pleated to form a series of adjacent channels, each channel located between opposed faces of each fold. The edges of the folds in the membrane are sealed together by potting the edges in a sealant. The membrane is then placed in a case usually comprised of polystrene, a styrene-acrylonitrile copolymer or a polycarbonate polymer wherein the chemical separation takes place. In the case of dialysis, the dialysis solution is placed on one side of the membrane and blood is placed on the other side. Polyepoxides and polyurethanes have generally been used to seal the edges of the folded membranes. U.S. Pat. No. 4,267,044 provides a thixotropic polyurethane composition which is particularly useful for sealing such folded membrane devices.
The polyurethane systems of the prior art which have been used to pot the ends of hollow fiber or folded membrane separatory devices have various limitations. For example, the fibers have to be dried prior to potting, otherwise, residual moisture will cause bubbling in the composition when contacting the polyurethane mixtures prior to cure. The drying process is costly and in some cases is not possible, for example, with fibers requiring a large amount of glycerine to sustain pore openings. In addition to the significant amount of moisture or water which exists in these devices, glycerine, which interferes with the reaction between isocyanates and polyols, is also present. The polyurethane systems of the prior art are not suitable for fibers containing great amount of moisture or glycerine. The present invention provides compositions which overcome these problems by utilizing specific hydroxyl bearing compounds and isocyanate compounds which accelerate the reaction of the composition, thus preventing bubble formation.